The prefix letter ל that is used with the infinitive in most cases teaches about the preparation of a matter - ויצאו ללכת ארצה כנען (and they went forth with them from Ur of the Chaldees, to go into the land of Canaan)²⁵⁵. When referring to the instrument with which one performs an action, the prefix ב is proper. And when it is written לבטא בשפתים (clearly with his lips)²⁵⁶, it was proper to say בביטוי שפתים, and there is a homiletical interpretation.

Leviticus 5:4 says,או נפש כי תשבע לבטא בשפתים להרע או להיטיב לכל אשר יבטא האדם בשבעה ונעלם ממנו והוא ידע ואשם לאחת מאלה (or if any one swear clearly with his lips to do evil, or to do good, whatsoever it be that a man shall utter clearly with an oath, and it be hid from him; and, when he knows of it, be guilty in one of these things), using לבטא בשפתים for “clearly with his lips”. If it had used the language בביטוי שפתים instead, one might have thought that it was also referring to someone who utters an oath but did not mean it in his heart; the language לבטא בשפתים teaches otherwise²⁵⁷.

footnotes: ²⁵⁵ Genesis 11:31. ²⁵⁶ Leviticus 5:4. ²⁵⁷ Malbim, HaTorah v’HaMitzvah Vayikra 311.